1. Field of the Invention
The present invention relates to a method for purging a fuel cell system, and more particularly, to a method for purging a fuel cell system capable of emitting combustible gas remaining in a fuel cell system outwardly at the time of stopping the fuel cell system.
2. Description of the Background Art
FIG. 1 is a schematic view showing a conventional fuel cell system of a proton exchange membrane fuel cell (PEMFC) method in which a hydrocarbon-based fuel such as LNG, LPG, CH3OH, gasoline, etc. is used as a fuel by refining only hydrogen by a desulfurizing process, a reforming process, and a hydrogen refining process.
As shown, the conventional fuel cell system comprises a fuel supply unit 10 for supplying only hydrogen extracted from LNG to a stack unit 30, an air supply unit 20 for supplying air to the stack unit 30 and the fuel supply unit 10, and a stack unit 30 for generating electricity by supplied hydrogen and air.
As a fuel and steam perform a reforming process in the fuel supply unit 10, hydrogen is generated. In order to generate the steam, the fuel supply unit 10 is provided with a steam generator 10b and a burner 10a for supplying heat to the steam generator 10b. 
In order to drive the burner 10a, a fuel is supplied to the burner 10a, and then off-gas remaining in the stack unit 30 after generating electricity is supplied to the burner 10a. 
When a signal for stopping an operation of the fuel cell system is applied to the fuel cell system, combustible gas (methane, city gas, hydrogen, etc.) remaining in the fuel supply unit 10 and the stack unit 30 is purged by using nitrogen thereby to be exhausted outwardly.
However, in case of purging combustible gas by using nitrogen, an additional container for storing the nitrogen is required and thus a fabrication cost is increased. Also, when the nitrogen is used up, new nitrogen has to be periodically contained in the container. Furthermore, when the fuel cell system is used at home, there is a difficulty in obtaining an additional space for installing the nitrogen container.